Peak to Average Power Ratio (PAPR) is one of the most critical problems in an Orthogonal Frequency Division Multiplexing (OFDM) system. In a radio front end, a high PAPR may lead to serious back-off and thus a low efficiency of a power amplifier (PA). To reduce the PAPR of an OFDM signal, a number of Crest Factor Reduction (CFR) techniques have been proposed. They can be classified into two categories, coding based CFR and clipping based CFR. The coding based CFR techniques cause no signal distortion, but have high computational complexity and low spectrum efficiency. When one Remote Radio Unit (RRU) is connected to multiple Digital Units (DUs), the coding based CFR techniques are not applicable and therefore only the clipping based techniques can be considered. The clipping based CFR techniques solve a problem that can be represented as: given an input signal, how to obtain an output signal with the lowest PAPR while satisfying requirements of Error Vector Magnitude (EVM) and Adjacent Channel Leakage Ratio (ACLR).
Noise Shaping (NS)-CFR (see X. Li, L. J. Cimini Jr, “Effects of Clipping and Filtering on the Performance of OFDM” in Proc. of Vehicular Technology Conference, IEEE, pp. 1634, 1997) is one of the most popular clipping based CFR algorithms. As shown in FIG. 1, a NS-CFR device extracts, from an input signal, a signal sample which exceeds a certain threshold and shapes it using a noise shaping filtering to obtain a shaped noise signal. Then, the shaped noise signal is subtracted from the original input signal.
For a better PAPR performance, a cascaded clipping scheme can be applied. Normally, four to six CFR stages are needed. The complexity of a NS-CFR device is mainly caused by the noise shaping filter. To reduce the complexity, a carrier de-combiner (including carrier transposing and down-sampling functions) can be provided following the NS-CFR device. An up-sampler may also be needed (see H. Chen and A. M. Haimovich, “Iterative estimation and cancellation of clipping noise for OFDM signals,” IEEE Commun. Lett., vol. 7, no. 7, pp. 305-307, July 2003).
Most samples of the noise signal in the NS-CFR algorithm are zeros. To improve the efficiency, a Peak Cancellation (PC)-CFR (see E. Hemphill, S. Summerfield, G. Wang, and D. Hawke, “Peak Cancellation Crest Factor Reduction Reference Design”, XAPP1033 (V1.0), Xilinx, Nov. 18, 2007) algorithm has been proposed. FIG. 2 shows a structure of a typical PC-CFR device. When a peak is detected from an input signal by a peak detector, the PC-CFR device allocates the peak to a peak generation unit (PGU) which then generates a pre-shaped and scaled cancellation peak to be subtracted from the input signal after being subjected to a matched delay. Since peaks are sparse in time, a small number of PGUs are generally sufficient to fulfill the clipping task. However, since the number of PGUs is limited, it is always possible that, when all PGUs are occupied, a new coming peak will be missed. This will lead to a problem of peak leakage and create a high risk for a PA. This problem can be mitigated by a cascaded clipping scheme. A typical implementation of PC-CFR has two to four clipping stages.
Compared with the PC-CFR algorithm, the complexity of the NS-CFR algorithm is higher while the required PAPR is not very low. However, the NS-CFR can guarantee that all peaks are eliminated. On the other hand, the PC-CFR is efficient in canceling high peaks with low EVM and ACLR costs, but its peak leakage is a fatal problem. That is, both algorithms have their own advantages and disadvantages. Thus, there is need for a CFR structure that can utilize the advantages of both CFR algorithms to obtain an improved performance in terms of PAPR, EVM and ACLR.
In Long Term Evolution (LTE) systems, the CFR techniques face a new challenge from increased information bandwidth and increased number of carriers. For a multi-carrier signal such as an OFDM signal, its PAPR increases with the increase of its bandwidth. The performance of the CFR algorithm highly depends on the sampling rate of the input signal. The sampling rate, and thus the complexity, will increase with the increase of the signal bandwidth. For example, if the signal bandwidth doubles, the complexity of the CFR algorithm increases by at least a factor of 2. The complexity of the CFR algorithm also increases with the increase of the number of carriers of the signal. Thus, a CFR algorithm or structure with low complexity is highly desired especially for a multi-carrier signal.
Additionally, as noted above, the peak leakage is a serious defect of the PC-CFR algorithm, especially for high peaks. If a high peak is leaked, the PAPR performance will be severely degraded. Therefore, it is desired that, if the peak leakage is inevitable due to limited number of PGUs in a PC-CFR device, the probability of high peak leakage can be minimized.